A secondary battery is a device that stores electrical energy in chemical form and converts the stored chemical energy into electrical energy to generate electricity when needed. The secondary battery is also referred to as a rechargeable battery because it can be recharged repeatedly. A common secondary battery includes a lead accumulator, a NiCd battery, a NiMH accumulator, a Li-ion battery, and a Li-ion polymer battery. When compared to a disposable primary battery, not only is the secondary battery more economically efficient, it is also more environmentally friendly.
A secondary battery is currently used in applications requiring low electric power, for example, equipment to assist in starting a vehicle, a mobile device, a tool, and an uninterruptible power supply. Recently, as the development of wireless communication technologies is leading to the popularization of mobile devices and there is a movement toward the mobilization of many types of existing devices, the demand for a secondary battery is dramatically increasing. Also, in the aspect of preventing environmental pollution, a hybrid vehicle and an electric vehicle have been developed, and these next-generation vehicles are employing technology for reducing the cost and weight and increasing the service life using a secondary battery.
Generally, most secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of a secondary battery that mounts an electrode assembly composed of a negative electrode, a positive electrode, and a separator in a cylindrical or prismatic metal can or a pouch-shaped case of an aluminum laminate sheet, and injects an electrolyte into the electrode assembly. Because a predetermined space for mounting a secondary battery is necessary, a cylindrical, prismatic or pouch shape of a secondary battery is a limitation in developing portable devices of various shapes. Accordingly, there is a need for a new type of secondary battery that is easily adaptable in shape.
To fulfill this need, suggestions have been made to develop a linear battery having a very high ratio of length to cross-sectional diameter. Korean Patent No. 0804411 discloses a linear battery including a plurality of negative and positive electrodes with separators interposed between the negative and positive electrodes, and Korean Patent No. 0742739 discloses an adaptable battery including positive electrode threads and negative electrode threads in the shape of a thread, but they have poor flexibility. Also, when deformation takes place in an outer current collector of a cable-type secondary battery by the outside force, a short circuit may occur by the contact with an inner current collector.
Also, in the manufacture of a linear battery, when a negative electrode and a separator are produced and a positive electrode is then coated on the electrode, inner coating layers of the separator and the negative electrode melt and ooze by an organic solvent included in a positive electrode slurry.
Also, in the manufacture of a general battery, when producing a positive electrode on a plane, an operation of compressing the electrode to a predetermined level is needed. This is to improve electric contacts between a positive active material/a conductive material/a current collector through the compression operation because the positive active material has low conductivity. However, in the case of a linear battery, according to its shape, because it is very difficult to perform an electrode compression operation after coating a positive electrode, an electric contact of an electrode gets loose, causing deterioration in life span of the battery.